PANTROPICAL SPOTTED DOLPHIN
Stenella attenuata (Gray 1846)

Order Cetacea : Family Delphinidae

DESCRIPTION. A small dolphin with a relatively short black beak, blackish back, grayish sides, and white underparts; eyes usually encircled with black rings joined by a black stripe across base of rostrum; dorsal fin, flipper, and flukes black; the pale sides and abdomen often covered with small blackish spots; posterior to the dorsal fin the blackish upperparts and the flippers often covered with grayish white dots. Teeth small (diameter at alveolus 2.5–3 mm), numbering 38–42 in each tooth row. Total length, 1.6–2.5 m. Similar to Atlantic spotted dolphins (S. frontalis) but upperparts blackish, general size smaller, beak narrower, and the teeth smaller and more numerous.

DISTRIBUTION. The distribution of pantropical spotted dolphins includes tropical and warm temperate waters around the world. It is the most abundant species of cetacean in the northern Gulf of Mexico, where it occurs in deep waters off the continental shelf. There are several stranding records from the coast of Texas, including two from the most recent stranding period (2002–2014).

SUBSPECIES. Subspecies designations have not been assigned for this species in most of its range.

HABITS. These dolphins are usually seen in groups of 5–30, although large herds of 1,000 or more are occasionally observed. Unlike many other dolphins, groups of pantropical spotted dolphins do not appear to be segregated by sex and age. These dolphins feed at or near the surface on fishes (including mackerel and flying fish), squid, and shrimp.

In the eastern tropical Pacific, the following reproductive data are known. The gestation period lasts 11.5 months and lactation lasts about 11 months. At birth, the calves average 80 cm in length and at 1 year are 1.4 m long. Males attain sexual maturity at about 6 years of age; females reach maturity at 5 years. The calving interval is 26 months. No data on reproductive habits are available for the Gulf of Mexico.

In the Pacific, these dolphins are killed incidentally in the course of seining for tuna. In 1970 about 400,000 were killed by US vessels alone, but that figure was reduced to 15,000–20,000 by 1978. Currently, incidental catch is limited by US law to 20,500 per year but is usually lower than that because of declining tuna-seining efforts and the recent adoption of a porpoise mortality reduction program; this international agreement among all major tuna-seining countries had a goal of reducing total incidental catch to fewer than 5,000 dolphins per year by 1999. In the Atlantic and Gulf of Mexico, the problem of incidental catch is limited and was never as great as in the Pacific.

POPULATION STATUS. Common; strandings and observations. Although pantropical spotted dolphins were once thought to be uncommon in the Gulf of Mexico, they are now known to be the most common cetacean in deep Gulf waters. It is probable that they were often misidentified in the past as Atlantic spotted dolphins. Stranding records exist all along the Texas coast as well as from Alabama, Mississippi, Florida, the Florida Keys, and Cuba. From the GulfCet surveys in the northern area of the Gulf, population estimates of almost 60,000 animals were derived, with most individuals occurring well beyond the 100 m (328 ft.) depth contour and into waters 2,000 m (6,562 ft.) deep.

CONSERVATION STATUS. The pantropical spotted dolphin is not considered a species of concern by the TPWD, USFWS, or IUCN.

REMARKS. The pantropical spotted dolphin can be confused with the Atlantic spotted dolphin, S. frontalis. Stenella frontalis is stockier, has dark spots ventrally (like subadult S. attenuata), and lacks the dark bridle line. Stranded specimens are difficult to distinguish if the body markings have faded. The two spotted dolphins have fewer total teeth than other species of Stenella, and the tooth counts for the two species are similar. Stenella frontalis has approximately 70 vertebrae whereas S. attenuata has about 80, and the two species are not known to overlap in this character.

 

From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.